1. Field of the Invention
The present invention relates to an apparatus for treating pulp. The apparatus is especially suitable for screening fiber suspensions in the wood processing industry and/or for thickening. In other words, the apparatus is used where either fibers, liquid or both are to be removed from fiber suspensions. Specifically, the apparatus in accordance with the present invention may be used, for example, for the treatment of reject from a paper machine or for knot separation subsequent to the blow tank in pulp digestion.
2. Description of the Related Art
The present invention is based on a so called Stocker screw, which is described, for example, in U.S. Pat. Nos. 3,947,314 and 4,160,722. Also, EP Patent 0 034 780 shows an apparatus of this type. Thus, an apparatus in accordance with the prior art has an upright or a slightly inclined shaft and an evenly ascending, continuous screw thread mounted on the shaft. The screw thread is used for upwardly conveying the material to be treated, which is introduced into the lower end of the apparatus. In addition, in the known apparatus, the screw thread is surrounded by a screen surface through which the accepted material is discharged from the lower end of the apparatus and the material rejected to the inside of the screen surface is discharged from the upper end of the apparatus. Furthermore, the apparatus is frequently used as a so called knot washer; in other words, the apparatus is used for separating fibers and fiber flocs entrained with the knots and like larger solid particles of the fiber suspension and accepting them through the screen surface. During this accepting process, the knot material is also dewatered, so that the consistency of the rejected material quickly increases. The purpose is to raise the consistency as high as possible, because the knot material discharged from the apparatus is mostly incinerated. Thereby, the water content of the pulp should, of course, be as low as possible. The consistency is limited in practice by the fact that, although all of the loose liquid in the pulp is removed, so much liquid is absorbed in the knots and like particles that the consistency will be only 30%.
An apparatus known from U.S. Pat. No. 3,616,932 includes means mounted to a helical flight of the screw feeder for feeding washing liquid to the screen surface. The means are formed, according to one embodiment, by a tube mounted to the periphery of the flight on the discharge side of the screw. The tube is provided with perforations through which the washing liquid is injected to the screen surface. However, this embodiment is not advantageous because, firstly, the manufacture of the apparatus is very difficult and, secondly, most of the liquid fed through the spiral tube will be used for the dilution of the pulp. By using the spiral pipe, it may be ensured that the liquid supply takes place homogeneously throughout the whole screen surface, which probably has been the objective of the construction, but practice has proved that this arrangement also has some disadvantages. Since the pitch of the screw is relatively small, the pitch of the helical tube is also small. However, washing liquid must be injected to the screen surface so that the washing jet hits each of the perforations in the screen surface. Therefore, the spiral tube must be provided with a rather dense network of feed perforations. A rather high amount of liquid is supplied to the screen surface, which results in a liquid film flowing on both sides of the screen surface, so that the consistency of the pulp to be treated is considerably diluted, especially on the upper part of the apparatus, in which part the apparatus is used to achieve the maximum consistency. However, so much liquid has to be fed in, in order to maintain the screen surface clean, that it flows downwards along the whole of the screen surface.
Further, the use of a continuous screw brings about problems, because the pulp to be treated tends to move as a uniform plug in an apparatus, such that the consistency thereof close to the shaft of the apparatus remains relatively small, whereas the consistency close to the screen surface may become very high.
A second arrangement worth mentioning is one in which it is an object not so much to wash the screen surface, but to dilute the reject material remaining inside the screen surface of the apparatus so as to wash off usable fibrous material from the material to be rejected and to allow it to be discharged through the perforations of the screen surface to the accept side. In this prior art arrangement, the dilution liquid is fed directly from the shaft, so that the dilution effect thereof is not directed to where it should be, in other words, to the thickened pulp layer, which is naturally located in close proximity to the screen surface.
However, the capacity of the prior art apparatuses is relatively small, because they tend to clog rather easily. It is clear that, in an apparatus which has a thread homogeneously conveying the suspension upwards and in which it is possible contemporarily to remove liquid for thickening and to remove fibers for screening, the flow takes place merely towards the screen surface, so that the openings thereof, of course, clog quickly. Further, it must be noted that the consistency of the suspension fed into the apparatus is at its lowest close to the shaft and increases to a maximum when approaching the screen surface. Thus, the clogging of the screen surface occurs also because of the increase of the consistency towards the screen surface, and the fibers which are already loose from the knots form a fiber matting on the screen surface. The sticking of fibers, fiber flocs and fiber matting on the openings of the screen surface is further caused by a grinding effect of the screw, by which the screw tends to remove the thickest fiber accumulations from the screen surface. At the same time as the screw scrapers off fiber matting from the surface, it presses a portion of the matting deeper into the openings of the screen surface. All the operations of the screw on the material to be treated are very gentle and continuous, so that the fiber matting does not tend to break, the flocs formed by the fibers do not break and the consistency of the material is not homogeneous. This is because the material rises in the tube defined by the screen surface practically as one uniform column.
As can be appreciated from the above, the greatest defect in the apparatuses in accordance with the prior art is their small capacity, which is due to the clogging of the screen surface.